1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to a circuit breaker for a secondary battery, which can shut out current and prevent explosion of the battery when internal pressure of the battery is increased above a permissible level, thereby increasing safety of the battery.
2. Description of the Prior Art
Generally, secondary batteries are compact in size and rechargeable, while providing a large capacity. Well known as secondary batteries are a Nickel metal hydride battery, a Lithium battery, and a Lithium(Li)-ion battery.
Particularly, the Li-ion battery uses lithium metal oxide as a positive active material and carbon or a carbon compounds as a negative active material. Lithium salt dissolved in an organic solvent is used as an electrolyte via which lithium ions move between positive and negative electrodes. Charging and discharging operations are realized while the lithium ions are moved between positive and negative electrodes.
FIG. 6 shows a conventional Li-ion battery.
A rolled electrode assembly 2 is inserted into a can 4 filled with electrolyte, the rolled electrode assembly consisting of positive and negative electrodes and a separator disposed between the positive and negative electrodes. Mounted on an upper end of the can 4 is a cap assembly 8.
The cap assembly 8 is insulated from the can 4 by a gasket 6. That is, the upper end of the can 4 is crimped on an outer periphery of the cap assembly 8 with the gasket 6 interposed therebetween. The cap assembly 8 includes a plate 14 provided with a safety groove, a current control member 12 disposed on the plate 14, and a cap cover 10 disposed on the current control member 12. Attached under the plate 14 is a circuit breaker 18 with an insulator 16 disposed therebetween. A positive tab 20 is welded to the circuit breaker 18 before the circuit breaker 18 is assembled in the cap assembly 8.
In the Li-ion battery, when the battery malfunctions or is overcharged, the electrolyte is dissolved at a positive electrode, while lithium metal is extracted from a negative electrode, causing a short circuit. As a result, internal pressure of the battery is increased above a permissible level. This may cause the battery to explode.
However, to prevent the above hazardous problems, the circuit breaker 18 is designed to be detached from the plate 14 to interrupt current flow when the internal pressure is excessively increased, thereby providing safety to the battery. Here, the detachment force of the circuit breaker 18 from the plate 14 is determined by adhesiveness of the weld between the circuit breaker 18 and the plate 14. Therefore, it is greatly difficult to precisely set the detachment force with respect to the internal pressure of the battery.
That is, if the welding force of the circuit breaker 18 to the plate 14 is set to be too high, since the circuit breaker may not be detached from the plate 14 even under the excessively increased internal pressure, the battery may explode. And if the welding force is too low, the circuit breaker may be detached from the plate 14 even under the permissible internal pressure, deteriorating working efficiency of the battery.
Therefore, the present invention has been made in an effort to solve the above described problems.
It is an objective of the present invention to provide a circuit breaker for a secondary battery, which can reliably prevent any hazardous problems caused by excessive internal pressure of the battery.
To achieve the above objective, the present invention provides a cap assembly comprising a plate, and a circuit breaker disposed under the plate. As a feature of the present invention, the circuit breaker comprises an attaching portion firmly attached to the plate and a mechanically weak portion formed around the attaching portion.
Attaching force of the attaching portion to the plate is higher than detachment force of the mechanically weak portion.
Preferably, the mechanically weak portion comprises a notch formed around the attaching portion. A depth of the notch is determined such that the notch can be broken by an internal pressure of 7-15 kgf/cm2.
Preferably, the mechanically weak portion of the circuit breaker is thinner than the remainder of the circuit breaker.
More preferably, the circuit breaker is provided with a centrally formed opening and a bridge formed across the opening such that the mechanically weak portion is notches formed on both ends of the bridge and the attaching portion is a portion between the notches.
Alternatively, a circular notch is formed around the attaching portion of the circuit breaker such that the attaching portion can break away from the remainder of the circuit breaker at the circular notch.
In addition, the present invention provides a circuit breaker for secondary batteries including a first portion for attaching to a plate, a second portion formed around the first portion, and a third portion connecting the first portion to the second portion, the third portion being broken under a predetermined level of pressure applied to the plate.